In terms of semiconductor technology, the image sensor device is used to sense the light projecting onto the semiconductor substrate. Common image sensor devices include complementary metal oxide semiconductor (CMOS) image sensor devices and charge coupled device (CCD) sensor devices, widely applied in all respects, for instance, in digital cameras. Those image sensor devices adopt a pixel array to receive light energy in order to transform an image into digital data. The above-mentioned pixel array may include photodiodes (PDs) and transistors. Currently, the structure of the CMOS image sensor device includes 3-T architecture and 4-T architecture. The 3-T architecture may include a reset transistor (RST), source follower (SF) transistor, and row select (RS) transistor etc., while the 4-T architecture may include a transfer transistor (TX), reset transistor (RST), source follower (SF) transistor, and row select (RS) transistor etc.
The transfer transistor (TX) belonging to the above-mentioned 4-T architecture serves to isolate the photodiode (PD) and the floating diffusion (FD) under an exposure operation or transfer charges from the photodiode (PD) to the floating diffusion (FD) under a readout operation.
Conventionally, the threshold voltage of the transfer transistor can be altered by process adjustment, for example, channel implantation doping, various thicknesses of gate dielectric layer or polysilicon implantation doping, etc. Generally, in the transfer transistor, the threshold voltage along the channel between the photodiode (PD) and the floating diffusion (FD) is uniform. While applying a higher operating voltage than the threshold voltage, the transfer transistor is turned on, and charges are transferred from the photodiode (PD) to the floating diffusion (FD). During this procedure, how to improve charge transfer efficiency is really important. On the other hand, while turning the transfer transistor off, how to prevent charges from leakage from the photodiode (PD) to the channel and to prevent residual charges in the channel from backflow to the photodiode (PD) is then important.
However, based on the conventional structural design of the transfer transistor, the potential energy distribution of the channel cannot be conducive to turn-on and turn-off statuses simultaneously.